1. Technical Field
This invention relates generally to a slot antenna and, more particularly, to a dual circular polarization double-layer slot array antenna which is capable of providing a dual circular polarized beam with optimum efficiency and bandwidth.
2. Discussion
Direct communication systems commonly employ antennas for transmitting and receiving energy between remote locations. Modernly, antennas are widely employed for an increasing number of applications, many of which require a low profile, wide bandwidth antenna that can operate with polarized radiating energy. For example, advanced Direct Broadcast Systems (DBS) are currently being developed for future generation cable television transmission. Currently, North America Direct Broadcast Systems are being developed which transmit circular polarized (CP) energy. These systems require low cost dual circular polarization eighteen inch aperture antennas at remote television locations for receiving the circular polarized signals via satellite transponders.
In the past, conventional reflector antennas were used which typically consisted of a reflector operatively coupled to a feed horn (polarizer) via a strout and an associated mounting structure. Such antennas include a Cassegrain antenna in which the feedhorn is displaced from the reflector at a focal point on the front side thereof. However, such conventional reflector antennas generally occupy a relatively large volume and are easily susceptible to damage from the environment.
More low profile antenna concepts have been developed which include planar slot antennas. One type of slot antenna includes a double-layer structure which forms two propagation layers. Double-layer slot antennas historically have included the excitation of a transverse-electromagnetic (TEM) mode travelling wave between a pair of parallel metallic plates. This type of slot antenna further involves radio frequency (RF) energy leakage through radiating slots formed on the upper metallic plate so as to form a boresight pencil beam. Such slot antennas have generally exhibited a relatively simple mechanical structure with potentially low fabrication costs. However, there are recognized limitations associated with the conventional slot antenna approaches. These limitations include the fact that either single feed designs or overly complicated multiple feed designs are generally employed to excite a pure TEM mode travelling wave between the parallel plates. While a number of feed design approaches have been proposed, the prior concepts are generally limited to a single polarization (CP or linear) or involve high complexity and exhibit low efficiency with a relatively narrow bandwidth.
Another type of slot antenna includes a radial line slot array antenna which has either a single or double layer structure with a plurality of coupling slots formed along spiral pattern. An example of one such radial line slot antenna is described in U.S. Pat. No. 5,175,561 issued to Goto. Such single-layer slot antennas have been employed for Direct Broadcast Systems in Japan and are generally capable of operating with single polarization energy only. That is, the radial line slot array may handle only either right hand or left hand circular polarization. An additional feed on another layer could be added to the single layer radial line slot array to provide dual circular polarization beams. However, the two beams would be dependent upon each other and optimization of one would degrade the other. That means if one circular polarized beam is optimized, then the other circular polarized beam will likely exhibit rather poor performance. As a consequence, the radial line slot array generally is not capable of effectively handling the combination of both right hand and left hand circular polarization, while achieving reasonably acceptable bandwidth and performance criteria.
It is therefore desirable to provide for a low profile planar dual circular polarization slot array antenna which overcomes limitations associated with the abovementioned prior art approaches. It is further desirable to provide for a double-layered slot antenna which is capable of operating with both right hand and left hand circular polarization and involves relatively low fabrication costs and less complexity, while maintaining high efficiency and wide bandwidth capabilities. In addition, it is further desirable to provide for such a slot antenna which exhibits two circular polarized beams which are optimized independent of one another.